javascript - js array map method, if the original array contains objects or arrays, the original array will also change. Why?

Question

var list = [{'a': 1},{'a': 2}];
var newList = list.map(function(index){
    return index.a += 1;
});
console.log(newList,'newList',list,'list');
// list也改变了 list = [{'a': 2},{'a': 3}]
// 本人小白，求大神指教，勿喷，谢谢！

Answer 1

It has nothing to do with map

js objects are reference types, characters and numbers are basic types

Basic type value transfer is copying

Reference type passing by value is a reference

For example:

var a = 1;
var b = a;
b++;
console.log(a);

and

var a = [1];
var b = a;
b[0]++;
console.log(a);

Answer 2

You first modify a single key value of the list, and then return the key value, naturally modifying two!